[c-nsp] Maximum spannig tree instances

[Jon Lewis]

On Tue, 14 Jul 2009, Tim Durack wrote:

> We left everything in MST0, and pull a few VLANs into MST2 for
> load-balancing reasons. Core-1 is root for MST0, Core-2 is root for MST2.
> Works for a simple topology, where every switch has redundant links back to
> a couple of core switches. Not sure it would be so great for the kind of
> topologies being discussed here.

The cisco examples I saw say to leave MST0 empty and use MST1 and MST2 for 
VLANs.

This concerns me though:

  Complete any MST configuration involving a large number of either
  existing or new logical VLAN ports during a maintenance window because
  the complete MST database gets reinitialized for any incremental change
  (such as adding new VLANs to instances or moving VLANs across instances).

Will adding new VLANs to an MST instance disrupt traffic flow for other 
VLANs in that MST instance?

The topology I have is actually 2 core switches with a bunch of edge 
switches redundantly uplinked to both cores.

> However, as soon as I want to add another VLAN to MST2, I have touch *every*
> switch in the MST region. And during the process MST is inconsistent -
> either I adjust the two core switches first, and every edge switch flips
> over to MST0, or I do every edge switch first, core last. Either way it's a
> lot of STP fun.

That sounds like another argument for rPVST and turning off VTP to avoid 
hitting the PVST instance limit on the less capable switches.

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  Jon Lewis                   |  I route
  Senior Network Engineer     |  therefore you are
  Atlantic Net                |
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